Device for screening pulp



Aug 11, 1959 A. G. SANDISON DEVIQCEIYFOR SCREENING PULP Filed April 18,1957 2 Sheets-Sheet 1 FIG. 1

INVENTOR A. G. SANDISON er: 7%: 4 ATTORNEYS fl- 11, 19 59 A. G. SANDISON2,899,069

DEVICE FOR SCREENING- PULP Filed April 18, 1957 v 2 Sheets-Sheet 2INVENTOR A. G. SANDISON er: 7&1 ATTORNEYS 2,899,069 DEV'ICEFOR SCREENINGPULP Alexander Greswolde' sandisou, Owen Sound, Ontario,

Canada,,assi gngr to {The William Kennedy. & Sons Limited, Owen Sound,Ontario, Canada, a corporation Application Ami-18,1957, Serial No.-653,553 6 cums. C1, 210 -405) This invention relates to devices for"screening pulp of the type usually referred to as centrifugal screensand, in particular, hasto dowithmeans for'applyin'g the pulp 'stoclt tothe screen element.

In screening devices of this type the unscreened stock is impelled andapplied with considerable tangential velocity to the interior ofacylindrical screen element, the 'velocity being sufficiently high tomaintain the stock against the screen element by the action ofcentrifugal force. It is commonpractice to impart the necessarytangentialvelocityto the stock by means of a rotor revolying Within thespace defined by the screen element, the rotor'usu'ally' beingconstructed with substantially radially extending yanes.

To obtain satisfactory output from screens of this type it is necessaryto employ a relatively high tangential velocity, velocities of theiorderof lO feet per second being commonly used. Considerable power isnecessaryto impart 'such'velocities to the 'stock, and the actual powerrequired naturally dependson the efficiency of'theimpelling" device,when consideredas a means for imparting energyof tangential elo'city'tdthestQck. The efiiciency considered may be more closely "defined 'as theratio of the energy of tangentialfldw' imparted tothe-s'tock per;i"secdn d to the ehergyper second supplied tothe-impeller.

It can be shown that wherea simple rotorof conventior'ial constructioniser'nployed to deliver the'stock i directly to the screen'surface thetheoretical limiting maximum value of the"efiiciency "so 'defin'ed isfifty percent, owing to unavoidable high shock'l'oss'es amounting to one1 half of the eriergy'supplied. 'This has been confirmed in practice,the actual efficiencies obtained being below 'fifty percent. Moreover,it "has been usual to regulate the fiow of stock to the screen"by"meansofa throttling valve in the'supply line sothat no effective'use is madeof the potential eiiergyfavailable from the pressure of supply.

An object of the present invention is to provide means wherebytangentialvelocity may be imparted to stock in acentrifiigal screenwithout incurring high shock losses, 1 thereby effecting a 'savingin thepower required.

Anothefobject'"ofthe invention is'to provide aconss'trllcticin' whrebythe'pote'ntial'energy of the supply pressure may,in considerablemeasure, be utilized in imparting tangential elocity to the'stock,'thereby making' possible' a further s aving inthe powerrequired.

A preferied embodiment ofthe invention is'described anddisclosed'here'in in detailwith'referen'ce to the accompanying-drawingsinwhichlike reference numerals refer tolike parts in the'various viewsand'inwhich:

Figure 1"is'a peispective view of a pulp screening device embod inthepresent invention with portions thereof cut awayto show the interiorconstruction;

Figure 2'is a viewtaken along line 22 of Figure lyand a, ,7

Figure 3 is a section taken along line"3 3 of Figure 1. Referring now toFigure l the pulp screening device -embodying the present invention willbe seen tobe of relatively conventional construction with respect to theextent of; the vanes l8 is suchthat the outer edges 18a 2,899,069Patented Aug. 11, 1959 "in ajor portions thereof. The device is providedwith an outer casing 10 to each end of which is secured an end wall 11.Ashaft l zisjournalled in bearings 13 and 14 and is provided, at oneend, with a pulley 15 by which it may be driven through the belt 16. Asleeve 17 surrounds and is secured to the shaft 12 between the end walls11 and, extending radially outwardly from the sleeve 17, are a seriesofIvanes 18 which serve to maintain the ';centrifugal velocity vof thestock within the screen in a manner which has been roughly outlinedabove.

Within the outer ,casing 10, concentric therewith and spaced therefromisa screen element 19 which is held in position by means ofmounting rings20 carried by the screen and flanges 21 carried by the end walls 11. Thescreen 19 is concentric with the'shaft'12 and the radial of the vanesapproach the'interior'surface of the screen element19 so as to act uponthe stock which is applied thereto as a whirling shell under the actionof centrifugal force. I

It will be {observed that the radially extending vanes 18 have portionscut -away adjacent one end wall Use as to'clear apipe' 22 which extendsinto the interior of the device to" deliver 'dilutingwater to the stockWithin the device bymeans ofholes23 as can be seen in Figure 1.

The invention with which the present disclosure is principally concernedis'er'nbodied in the construction adjacent the endw'all ll'remote fromthe diluting water pipe "22. 30

-se"parating the centrifugalimpeller from the radial vanes,

'It is appreciated that the sleeve 17 and the vanes 18 also constitute asort of centrifugal impeller but for the purposes"of"the'presentdescription and the claims it is "-to'be understood-that when the wordscentrifugal impeller are used it is intended to mean that impellerdefined by'the wall 25"and the vanes '24. p

The er'id "wall' 1 1' is formed with a'central aperture 26 through which-the'shaft 12 passes, the edges of the opening 26 being "extend ed inadirectio'n axially of the shaft to define -an elbow 27 which," at itsfree end, is fitted with a"fla'nge"2 8' to whicha'source of unscreenedpulp may be 'eorineaed m'suppI this"materia1'to'-the interior of theimpeller.

It will thus be s'een that iffiuid pulpis supplied to the elbow "27-"byineans 'ofthe inlet 29 defined'by the flange 28, the fluid will enterthe centrifugal impeller and be impelled radially outwardly by means ofthe vanes 24 when the shaft-12 is rotated'by means of pulley 1S andbelt' l6. Thera'dialextent of the vanes 24 of the centrifugal impellerisnot as great as the radial extent of the vanes 18 of the rotor withinthe body of the screening The wall '25, however, extends radiallyoutwardly further than the tips24a of the vanes 24 to define :an annularpocket surrounding the periphery of the vanes lfl. Within this pocketare mounted a series of guide vanes"30 which are pivoted to the endplate 11 by means of' shaft 31 which lie in a direction transverse tothe chord of each guide vane. Each of the shafts 3l passcompletelythrough bushings in the end Wall 11 wand, .exter nally ofthe end wallllare fitted with crank arms.:32- each of which is 'keyed to its shaft 31and each 1 ofnwhich is'.providedwitha stub shaft 33 extending inthesameTdirectionfrorn the crank arm -32 as the shaft 31 extends fromtheguide vane 30.

A,notched. -1;ing 34r is mounted externallyof the end wallll and cncentric with the, shaft 12 and elb0w' 27,

, each notch 35'receiving1one stub shaft 33 of one crank arm 32. Thestub shafts 33 are mounted in blocks 36 3 which are a sliding fit in thenotches 35 and which are a pivoting fit about the stub shafts 33. Theinternal diameter 37 of the ring 34 is a sliding fit around the portionof the elbow 27 immediately adjacent the end wall 11 and the ring 34serves to link all the crank arms 32 together. I i

A radially extending arm 38 is provided on the ring 34 and, at itsradially outer extremity, is pivotally secured to a block 39 by means ofa pin 40. The block 39 is secured to a rotatable threaded shaft 41 whichis in threaded engagement with a bushing 42 pivotally mounted on abracket 43. A hand wheel 44 is fitted and keyed to the end of shaft 41remote from block 39. Accordingly, it will be seen that by rotating thehand wheel 44 the shaft 41 can be moved in a direction parallel to itsown axis relative to the bracket 43 and can thereby cause the rotationof the ring 34 relative to the end wall 11. This will cause the angulardisplacement of the crank arms 32 and will result in a similar angulardisplacement of the shafts 31 and will, as a result, vary the angularposition of the guide vanes 30 relative to the tips of the impellerblades 24.

Referring now to Figure 3 it will be seen that the guide vanes 30, byvirtue of their configuration define a series of convergent outletnozzles for the centrifugal impeller comprising the vanes 24. Thedimension shown by the arrow A is larger than the dimension shown by thearrows B and, as a result, fluid which flows between adjacent guidevanes 30 in the direction of the arrow C will experience a drop inpressure and an increase in velocity. The degree of convergence of thenozzles defined by the guide vanes 30 can, of course, be varied byrotating the hand wheel 44 in one direction or another to increase ordecrease the distance represented by the arrow B.

It will also be seen that the impeller vanes 24 of the centrifugalimpeller are provided with a curvature to provide a slight forwarddischarge towards the inlets of the convergent nozzles defined by theguide vanes.

Having now described the construction of the preferred embodiment themethod of operation will be set forth.

Assuming that the belt 16 is in engagement with an operating prime mover(not shown) and that the shaft and the centrifugal impeller are rotatingat operating speed, pulp may be delivered to the inlet 29 defined by theflange 28 of the elbow 27. This will deliver fluid pulp to thecentrifugal impeller and the pulp will be impelled radially outwardly bymeans of the vanes 24. The curvature of the vanes 24 will discharge thepulp with both a radial and a tangential velocity into the inlets 45 ofthe convergent outlet nozzles defined by the guide vanes 30. As thefluid passes in the direction of the arrow C (Figure 3) its velocitywill be increased due to the convergence between the adjacent faces ofadjacent guide vanes and its direction will be changed so that when itleaves the convergent nozzles the direction of flow will besubstantially tangential to the screen element 19. The impellerdiameter, as measured across the tips of the vanes 24 should be sorelated to the diameter measured across the tips of the vanes 18 thatthe tangential velocity of the stock issuing from the nozzles defined bythe vanes 30 will be closely matched to the peripheral speed of thevanes 18 of the rotor within the body of the screening device. As aresult, the stock will be applied to the screen 19 substantiallytangential thereto and at a speed which will minimize the shockexperienced by the stock when it leaves the guide vanes and is actedupon by the vanes 18. The quantity of flow of the pulp through thescreening device can be varied at will by rotating the hand wheel.

Once the stock has been applied to the screen 19 through the nozzlesdefined by the guide vanes 30 its tangentlal velocity is maintained bythe @Qfion of the vanes 18 which are also rotated by the shaft 12. Thatportion of the stock which passes through the screen 19 will enter thespace 46 between the external surface of the screen element 19 and theinternal surface of the outer casing 10. From here the screened stockwill pass outwardly through an opening 47 in the casing which will beconnected to a conduit delivering the screened pulp to a storage vessel.The fibres of the pulp which are too coarse to pass through the screenelement 19 will be discharged from the screen element through an exhaustport 43 which will also be connected to a conduit to deliver therejected pulp to a suitable storage vessel. In order to maintain thewater content of the stock under the action of the screening device at asufficient level to promote elficient screening, a pipe 22 has beenfitted in end wall 11 remote from the inlet 29, the pipe being providedwith openings 23 through which diluting water may be admitted to theinterior of the screening device.

A construction of a pulp screening device having a centrifugal impellerassociated therewith has been described which enables the unscreenedpulp to be applied to the screen in a substantially tangential directionwithout encountering high shock losses. In addition, it will beappreciated that the pressure of the pulp supply will assist inimparting a velocity to the stock as it passes through the centrifugalimpeller. Obviously, a high head of pressure on the stock enteringthrough the inlet 29 will add to the pressure in the space between thevanes 24 and the vanes 30 and this pressure energy will, in turn, beconverted to tangential velocity energy as the stock passes through theconvergent nozzles defined by the guide vanes 30.

While a preferred embodiment of the invention has been described indetail with reference to the accompanying drawings, this is intended tobe illustrative rather than limiting and minor modifications may be madein the construction and arrangement of parts without departing from thespirit of the invention or the scope of the appended claims.

What I claim as my invention is:

1. In combination with a device for screening pulp, the device includinga screen element of the shape of a surface of revolution; a centrifugalimpeller mounted for rotation within one end of the screen element aboutan axis coaxial with the screen element, means to deliver pulp to theimpeller and a series of circumferentially spaced guide vanes positionedabout the periphery of the impeller to define convergent outlet passagesfor the pulp acted on by the impeller, the vanes being directed so thatthe pulp passing through the outlet passages will be applied to thescreen element in a direction substantially tangential to the screenelement.

2. In combination with a device for screening pulp, the device includinga screen element of the shape of a surface of revolution, a centrifugalimpeller mounted for rotation within one end of the screen element aboutan axis coaxial with the screen element, means to deliver pulp to theimpeller and a series of circumferentially spaced guide vanes adjustablymounted on a member stationary relative to the impeller and positionedabout the periphery of the impeller to define convergent outlet passagesfor the pulp acted on by the impeller, the vanes being directed so thatpulp passing through the outlet passages will be applied to the screenelement in a direction substantially tangential to the screen element.

3. In combination with a device for screening pulp, the device includinga screen element of the shape of a surface of revolution, a centrifugalimpeller mounted for rotation within one end of the screen element aboutan axis coaxial with the screen element, means to deliver pulp to theimpeller and a series of circumferentially spaced guide vanes adjustablymounted on a member statipnary relative to the impeller and positionedabout the periphery of the impeller to define adjustable convergentoutlet passages for the pulp acted on by the impeller, the vanes beingdirected so that pulp passing through the outlet passages will beapplied to the screen element in a direction substantially tangential tothe screen element.

4. In combination with a device for Screening pulp, the device includinga screen element of the shape of a surface of revolution, a centrifugalimpeller mounted for rotation within one end of the screen element aboutan axis coaxial with the screen element, means to deliver pulp to theimpeller and a series of circumferentially spaced guide vanes adjustablymounted on a member stationary relative to the impeller by means of apivot normal to the chord of the vane and parallel to its axis, thevanes being positioned about the periphery of the impeller to defineadjustable, convergent outlet passages for the pulp acted on by theimpeller, the vanes being directed so that pulp passing through theoutlet passages will be applied to the screen element in a directionsubstantially tangential to the screen element.

5. In combination With a device for screening pulp, the device includinga screen element of the shape of a surface of revolution, a centrifugalimpeller mounted for rotation within one end of the screen element aboutan axis coaxial with the screen element, means to deliver pulp to theimpeller, and a series of circumferentially spaced guide vanesadjustably mounted about the periphery of the impeller by means of pivotpins which are normal to the chords of the vanes and parallel to theiraxes, each pivot pin being journalled in a member stationary relative tothe impeller and having a crank arm on the end thereof remote from thevane, means linking each crank arm with all the other crank arms toenable all the vanes to be adjusted in unison, the guide vanes definingadjustable convergent outlet passages for the pulp acted on by theimpeller, the vanes being directed so that pulp passing through theoutlet passages will be applied to the screen element at a high velocityand in a direction substantially tangential to the screen element.

6. A device as claimed in claim 5 in which the means linking each crankarm With all the other crank arms includes a ring having a series ofradially extending slots in its outer periphery, each slot receiving onecrank arm and means to adjust the angular position of the ring relativeto the member stationary with respect to the impeller.

References Cited in the tile of this patent UNITED STATES PATENTS990,284 Miller Apr. 25, 1911 2,341,974 Browne Feb. 15, 1944 2,663,424Bynum Dec. 22, 1953

